1. Field of Invention
This invention relates to electronic ballasts for fluorescent lamps and, in particular, to electronic ballasts which stop operating in response to a fault condition such as a defective lamp or a missing lamp.
2. Prior Art
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Since a frequency much higher than 50-60 hz. is used, the inductors for an electronic ballast can be much smaller than the inductors for a magnetic ballast.
Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. The voltage on the capacitor is not constant but has a 120 hz "ripple" that is more or less pronounced depending on the size of the capacitor and the amount of current drawn from the capacitor.
Some ballasts include a boost circuit between the rectifier and the inverter. As used herein, a "boost" circuit is a circuit which increases the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt input) to 300 volts or more for operating a lamp, and/or which provides power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are sinusoidal and in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one).
If a lamp is not connected to an electronic ballast while power is applied to the ballast, the voltages and currents within the ballast can become extremely high, destroying the ballast. In addition, if a lamp is disconnected from a ballast, the person disconnecting the lamp is exposed to the high voltages of the ballast, e.g. by touching the terminals at one end of the lamp while the other end of the lamp is connected to the ballast. Many ballasts are designed to generate extra high voltages initially, to assure an instantaneous or a rapid start of a lamp, then to reduce the voltage when the lamp is conducting. When a lamp is removed, the circuitry within such ballasts reverts to a start-up mode and produces an extra high output voltage at the very time a person may be touching the terminals of the lamp.
Some electronic ballasts include a transformer in the output stage to isolate the lamp circuit from electrical ground. If a person touches the end of a lamp as he removes it, current cannot flow from the ballast through the lamp and through the person to electrical ground. An isolation transformer makes the ballast heavy and expensive. In addition, if a lamp is removed from such a ballast, the ballast typically reverts to a start-up mode which consumes large amounts of power, electrically and thermally stressing the components of the ballast.
In order to avoid stresses on the ballast, many circuits have been proposed for automatically shutting off the ballast when a fault condition is detected, e.g. a defective lamp or a missing lamp. U.S. Pat. No. 4,507,698 (Nilssen) discloses adding a ground fault interrupter to a ballast. A ground fault interrupter detects current flowing out of the ballast and returning by way of electrical ground rather than through the output terminals of the ballast. Since only a fraction of the current may return this way, the detection circuitry must be quite sensitive. Precise components must be used to avoid false triggering of the interrupter and these components significantly increase the cost of a ballast.
Other electronic ballasts include circuitry for monitoring voltage or current within the ballast and for shutting off the ballast when a fault is detected. A problem with such circuitry is that shutting off the ballast does not mean that the fault is corrected. Some ballasts resolve this problem by requiring that the applied power be turned off and then on in order for the ballast to restart, i.e. the ballast turns off and remains off until power is removed, starting normally when power is applied. Other ballasts enter a start mode after a predetermined length of time, typically a few seconds, and then periodically attempt to restart until turned off or until the fault is corrected.
Some ballasts do not turn off completely but only shut off the inverter. Other ballasts, if they have a voltage boost circuit, also shut off the boost circuit. If a fault is detected, it is desired to minimize power consumption by shutting off as much of the ballast as possible. This often requires a large number of components, increasing the cost of the ballast and often increasing power consumption when the ballast is operating normally.
A boost circuit and the inverter can each be self-oscillating, triggered, or driven. A driven circuit requires a source of pulses for operation and the pulses are provided by a timer circuit or a more complicated integrated circuit designed for ballasts or electronic power supplies. A triggered circuit typically incorporates a small pulse generator for starting the circuit into oscillation. A capacitor charging up to the firing voltage of a diac or other semiconductor switch is typically used in such circuits. The pulse generator may or may not be disabled when the ballast is operating normally. A self-oscillating circuit is constructed in such a way that the applied voltage causes the circuit to begin oscillation and typically includes a resistor having a high resistance to provide a temporary bias for initiating oscillation.
U.S. Pat. No. 4,562,383 (Kirscher et al.) discloses an electronic ballast in which a driven boost circuit is coupled to a triggered inverter for synchronous operation. An auxiliary winding on an output transformer senses excess voltage and triggers an SCR into a latched state to disable the inverter and the boost circuit if a lamp is removed. A disadvantage of a latched SCR is the continuous holding current through the SCR which causes unnecessarily high power dissipation and requires the use of expensive, high power resistors in the ballast. The coupling between the boost circuit and the inverter limits the amount of power factor correction which can be obtained from the ballast.
U.S. Pat. No. 4,554,487 (Nilssen) discloses an electronic ballast including a triggered inverter in which a portion of the inverter circuit is short circuited in the event of excess voltage across the output terminals. The inverter stays off until power is removed and then reapplied. This approach is impractical for commercial applications, e.g. re-lamping a department store or office building would entail turning off all of the lights seriatim.
U.S. Pat. No. 5,117,161 (Avrahami) discloses a self-oscillating inverter having two series connected switching transistors operating in push-pull. A resistor is also connected in series with the switching transistors. If the voltage drop across the series resistor exceeds a predetermined amount, a flip-flop circuit is set and the output signal from the flip-flop circuit causes a transistor to short circuit a portion of the inverter, quenching oscillation. An external signal is required for re-starting the inverter.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast which automatically shuts off in the event of a fault without dissipating large amounts of power.
Another object of the invention is to provide an electronic ballast including automatic shut-off circuitry which dissipates very little power either during a fault condition or during normal operation of the ballast.
A further object of the invention is to provide an electronic ballast which includes minimal circuitry for shutting off the ballast in the event of a fault and which requires no additional circuitry for re-starting the ballast when the fault is corrected.
Another object of the invention is to provide an electronic ballast which uses a boost circuit to provide a low voltage for operating integrated circuits within the ballast.
A further object of the invention is to provide an electronic ballast in which a brief displacement current through a capacitor in series with the lamp filaments starts a self-oscillating boost circuit and turns on the ballast.
Another object of the invention is to provide an electronic ballast which enters a quiescent state when a fault is detected, thereby drawing little or no power.